In 2019, NASA will send a capsule called Orion on an elaborate 25-day trajectory. First, the Space Launch System, the most powerful rocket ever built, will blast it into the ether. Then the capsule will coast 245,131 miles away from Earth, loop-de-loop around the moon, and scream back into Earth's atmosphere at 24,500 miles an hour. In the early 2020s, NASA plans to do the same thing again but with a crew—that mission will send humans farther into space than ever before. It's one small step in a decades-­spanning effort to send astronauts to explore asteroids, Mars, and beyond.

NASA gave photographer Vincent Fournier exclusive access to the testing and preparations for the mission, and our photographer spent 20 days at five facilities to capture how engineers build and test (and test, and test) the unprecedentedly large rocket and its human-carrying capsule. Engineers model everything from the orientation of rocket parts during transit to the way engine vibrations affect other components of the launch system. They're building teeny models of the rocket and sticking them in wind tunnels; enlarging the agency's trusty barge Pegasus to ferry massive hunks of metal from NASA's Michoud facility in Louisiana to Stennis Space Center in Mississippi and finally to Kennedy Space Center in Florida; and testing the fuel tanks by using hydraulic cylinders that apply millions of pounds of crushing forces to mimic launch and flight. "You know 'measure twice, cut once'?" says Andy Schorr, a manager of the rocket's payload integration at NASA. "We take that to a whole new level." Here's what goes on before the rocket goes up.